1. Field of the Invention
The present invention relates to a frequency control apparatus, a wireless communication apparatus, and a frequency control method.
2. Description of the Related Art
Carrier drift in a communication system significantly degrades the transmission quality. The carrier drift causes deformation of transmission signals and degradation of the frequency characteristic and the error rate in the transmission characteristic in a pass band.
There is a technology which detects phase data of symbols in a received signal and performs frequency control based on the detection of the phase rotation between symbols in order to prevent such degradation. With regard to the technology which performs frequency control based on the detection of the symbol phase, there are techniques of detecting the rotation of the phase by oversampling symbols at a rate higher than a symbol rate (see, for example, Unexamined Japanese Patent Application KOKAI Publication No. H6-261089 and Japanese Patent No. 3353724).
The system disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H6-261089 has a problem that it cannot cope with a large frequency error. As the system disclosed in Japanese Patent No. 3353724 uses the fact that if an intersymbol phase difference is a specified phase difference (either ±45° or ±135° for π/4 shift QPSK (Quadrature Phase Shift Keying)), the average converges to ±0°, the system takes time in detecting a frequency error, making fast control difficult, and has a difficulty in accurately detecting a phase error if there is an offset in the direction of phase deviation.